Field of the Invention
The present invention pertains to a downhole seismic event sensing system comprising a series of spaced-apart sondes which are conveyed into a wellbore on a fluid conducting coiled tubing string having electrical signal conducting cable associated therewith.
In recording seismic and other acoustic signals generated in an earth formation, it is sometimes advantageous to provide for signal receiving means disposed in a wellbore penetrating the formation. A long-standing problem prohibiting the development of downhole seismic arrays, particularly for use in oil and gas exploration, has been the limited power that can be converted downhole such as that available to be conducted by electrical cable or wireline. Since a relatively large number of seismic sondes would have to be interconnected and firmly locked against the wellbore wall, it would take considerable power to achieve the locking function at substantially the same time or, conversely, a considerable amount of time would be required to lock each sonde into engagement with the wellbore wall sequentially if a substantial amount of power was not available downhole. Moreover, the limited power transmission capability of conventional multi-conductor electrical cables or wirelines and the time required to lock the sondes sequentially has been heretofore been a disadvantage to the development of downhole seismic arrays.
However, with the continued development of systems and methods for extending coilable metal tubing into a wellbore, which tubing may convey high pressure hydraulic fluid as well as electrical signals through a cable disposed within the tubing, certain systems can be developed which have heretofore been unfeasible. An arrangement for conveying coiled tubing with an electrical conductor disposed therein into a wellbore for operating such devices as logging tools and the like is described in U.S. Pat. No. 4,685,516 to Lonnie J. Smith, et al., and assigned to the assignee of the present invention. A coiled tubing and wireline cable such as arrangement in accordance with the Smith, et al. patent, together with the features of the present invention, provides an improved downhole seismic array system.